The present invention relates to a cold start control system and method for use in a coherent lightwave communications system.
The coherent lightwave communications system can improve the sensitivity in reception by 10 to 20 dB over the conventional intensity modulation/direct detection (IM/DD) and facilitates optical frequency-division multiplex transmission. Therefore, the system is expected to be put to practical use in large capacity and long distance transmissions.
In the heterodyne receiver generally used in the coherent lightwave communications system, it is arranged such that a local light beam is mixed with a received signal light beam to obtain an intermediate-frequency signal (IF signal). The mixing of the signal light and the local light can be achieved by optically detecting the signal light and local light with a photodiode or the like having a nonlinear detecting characteristic. The IF signal thus obtained is demodulated to a base band signal.
Since the frequency of the IF signal corresponds to the difference between the frequency of the signal light and the frequency of the local light, when the center frequency of the signal light and/or the frequency of the local light varies with changes in ambient conditions or the like, the center frequency of the IF signal also varies. A variation in the center frequency of the IF signal injures normal demodulation. Therefore, in the heterodyne receiver, automatic frequency control (AFC) is executed, in general, to keep the center frequency of the IF signal constant.
The capture range of the AFC circuit is around 10 GHz at most. Against this, the oscillation frequency of a semiconductor laser serving as the light source of the signal light or the local light has a variation of several hundred GHz as a difference among individual products. Therefore, in cold starting a system, it is required that the IF signal be controlled to come within the capture range of the AFC circuit.